Scotland's First Minister Alex Salmond reacts as he concedes defeat in the independence referendum at the "Yes" Campaign headquarters in Edinburgh, Scotland on Friday. Later Friday Mr. Salmond announced he would be leaving his post as first minister and as head of the Scottish National Party. Reuters

EDINBURGH— Alex Salmond announced on Friday that he would stand down as leader of the Scottish National Party after he failed to convince a majority of Scots to vote to become independent from the U.K.

Mr. Salmond, speaking Friday afternoon in Edinburgh, said he wouldn't accept nomination for leader at his party's conference in November and that he would quit his role as the First Minister of Scotland's semiautonomous government once a successor is nominated. Earlier Friday, results from a historic referendum on Scotland's independence showed 55% voted against seceding from the union and 45% voted in favor of it.

The news of Mr. Salmond's resignation came as a shock, even to some of his close colleagues. One of his aides said his departure leaves a lot of question marks, such as who is going to handle Scotland's negotiations with the U.K. Parliament over the new governing powers promised. In the final weeks of the referendum, U.K. government leaders agreed to give Scotland more power over spending and welfare.

The Scottish Government didn't immediately respond to comment about what Mr. Salmond's resignation would mean for the government going forward.

Mr. Salmond, whose dream for decades has been to lead Scotland to independence, said in a speech Friday that he was proud of the 1.6 million people who voted for secession from the U.K. He also commended the remarkably high voter turnout in the referendum.

"I believe that in this new exciting situation, redolent with possibility, Party, Parliament and country would benefit from new leadership," Mr. Salmond said.

Mr. Salmond's tremendous political skills, charisma and determination brought him from a being minor player in U.K. politics to being a force that almost broke up a 307-year-old union. After the announcement, Prime Minister David Cameron said Mr. Salmond was an effective leader who always fought for his beliefs.

"While we disagree profoundly about his goal of a separated Scotland, and many other things, I respect and admire his huge contribution to politics and public life," Mr. Cameron said.

Since Mr. Salmond entered politics, he has seized every chance he has to fight for independence. As a student in the 1970s, he went door-to-door speaking with people about Scottish independence—even in the depths of winter—and drove distances to hand out newspapers with pro-independence messages.

Mr. Salmond was first elected as a member of the U.K. Parliament at Westminster in 1987, when the Scottish National Party was a fringe party. It only had three SNP lawmakers among 650 in the House of Commons.

Over the years, Mr. Salmond rose through the party's ranks and led the SNP to victory in 2007 and then again in 2011, when the SNP won an unexpected majority at the Scottish Parliament. His calls for independence gained popularity among Scots, many of whom felt disillusioned and distrustful of politicians at the U.K. Parliament.

Ruth Davidson, a Scottish member of parliament for the Conservative Party, said she applauded Mr. Salmond for stepping down and that his resignation would help Scotland reunite after the referendum.

"While the referendum campaign has been hugely invigorating, by its very nature it has divided too," Ms. Davidson said in a statement.

The Scottish National Party declined to comment. Mr. Salmond said he would continue to offer to serve as a member of the Scottish parliament after his resignation.

Write to Nicholas Winning at nick.winning@wsj.com and Jenny Gross at jenny.gross@wsj.com